feat(windows): mpv plays audio on Windows, with libmpv shipped in the installer

libmpv on Windows (DR-237):
- The builder image carries zhongfly's LGPL libmpv-2.dll, pinned by
  asset name and sha256, plus an MSVC mpv.lib generated from the DLL's
  own mpv_* exports (the archive ships only a MinGW .dll.a). LGPL, not
  the GPL builds: no x264/x265, mpv -Dgpl=false; FFmpeg is version3, so
  LGPL-3.0. THIRD_PARTY_NOTICES.md records it.
- build-windows-cross.sh stages both files into src-tauri/windows-libs/;
  build.rs links mpv.lib from there and tauri.windows.conf.json bundles
  the DLL beside jellytau.exe from there, with the licence texts under
  licenses/. One directory, so the DLL shipped is the one linked.
- Workflows move to builder image 2026.09.1.

Windows audio:
- MpvBackend replaces WebviewAudioBackend on Windows (ao=wasapi), so
  volume, EQ, normalization and gapless work there as on Linux. Local
  files are passed to mpv as native paths, not file:// URLs.

Fixes found on the way:
- player_play_item decided "does the backend render video" with
  cfg!(not(linux)), true on Windows, while get_player_status sent
  Windows video to the <video> element. With mpv as the backend that
  would decode every film's soundtrack twice. All three callers now
  ask video_renders_natively() (UT-273).
- confine_queued_path rebuilt paths with PathBuf::push, so on Windows
  a queued `downloads/x` was stored as `downloads\x`, no longer the
  spelling the app built. It now keeps the caller's separator (UT-205,
  which only ever ran on Linux, failed under Windows).

Verified: jellytau.exe imports libmpv-2.dll; the full unit suite
cross-compiled for Windows passes under wine against the shipped DLL
(943/943, including the mpv injection and TLS tests); the NSIS
installer contains the DLL and licence texts. Not yet run on real
Windows hardware.

Dockerfile.builder

@@ -186,6 +186,42 @@ RUN . $HOME/.cargo/env && \
     cargo cyclonedx --version && \
     mdbook --version
 
+# ---------------------------------------------------------------------------
+# libmpv for Windows (DR-237) — the Windows build links it and ships the DLL.
+#
+# zhongfly/mpv-winbuild's **LGPL** dev build: libmpv-2.dll with FFmpeg compiled
+# in, built `-Dgpl=false` with no x264/x265 and no --enable-gpl (its
+# compile-lgpl-libmpv.patch). FFmpeg keeps --enable-version3, so the DLL is
+# LGPL-3.0: the installer ships the licence texts and keeps the DLL a separate,
+# replaceable file — THIRD_PARTY_NOTICES.md. Not the default (GPL) asset from
+# the same release, and not shinchiro's, which is GPL-3.0 only.
+#
+# Pinned by asset name *and* sha256: GitHub prunes old releases from that repo,
+# so a rebuild after pruning fails loudly here rather than quietly taking a
+# newer build. To bump, change all three values.
+#
+# The archive ships a MinGW import library (libmpv.dll.a); the MSVC target needs
+# `mpv.lib`, generated from the DLL's own mpv_* exports so it cannot name a
+# symbol the DLL lacks. Output: $LIBMPV_WIN_DIR/{libmpv-2.dll,mpv.lib,include}.
+ENV LIBMPV_WIN_RELEASE=2026-09-24-2a4eb8067c \
+    LIBMPV_WIN_ASSET=mpv-dev-lgpl-x86_64-20260924-git-2a4eb8067c.7z \
+    LIBMPV_WIN_SHA256=f89c6195e13bfce3e8c0cc5d7f5d889ebdcf12184a41c8288360f5acd1e7306c \
+    LIBMPV_WIN_DIR=/opt/libmpv-win64
+RUN apt-get update && apt-get install -y --no-install-recommends libarchive-tools \
+    && rm -rf /var/lib/apt/lists/* \
+    && wget -q "https://github.com/zhongfly/mpv-winbuild/releases/download/${LIBMPV_WIN_RELEASE}/${LIBMPV_WIN_ASSET}" \
+        -O /tmp/mpv-dev.7z \
+    && echo "${LIBMPV_WIN_SHA256}  /tmp/mpv-dev.7z" | sha256sum -c - \
+    && mkdir -p "$LIBMPV_WIN_DIR" \
+    && bsdtar -xf /tmp/mpv-dev.7z -C "$LIBMPV_WIN_DIR" libmpv-2.dll include \
+    && rm /tmp/mpv-dev.7z \
+    && { echo "LIBRARY libmpv-2.dll"; echo "EXPORTS"; \
+         llvm-readobj --coff-exports "$LIBMPV_WIN_DIR/libmpv-2.dll" \
+           | awk '/Name: mpv_/ { print "    " $2 }'; } > "$LIBMPV_WIN_DIR/mpv.def" \
+    && llvm-lib /def:"$LIBMPV_WIN_DIR/mpv.def" /out:"$LIBMPV_WIN_DIR/mpv.lib" /machine:x64 \
+    && test "$(grep -c '^    mpv_' "$LIBMPV_WIN_DIR/mpv.def")" -ge 50 \
+    && ls -la "$LIBMPV_WIN_DIR"
+
 WORKDIR /app
 
 ENTRYPOINT ["/bin/bash"]